I. Field of the Invention
The invention generally relates to digital telephone systems and in particular to techniques for detecting bad data packets.
II. Description of the Related Art
FIG. 1 is an illustrative block diagram of a variable rate CDMA transmission system 10 described in the Telecommunications Industry Association's Interim Standard TIA/EIA/IS-95-A Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System. This transmission system may be provided, for example, within a base station of a cellular transmission system for use in transmitting signals to mobile telephones within a cell surrounding the base station.
A microphone 11 detects a speech signal which is then sampled and digitized by an analog to digital converter (not shown). A variable rate data source 12 receives the digitized samples of the speech signal and encodes the signal to provide packets of encoded speech of equal frame lengths. Variable rate data source 12 may, for example, convert the digitized samples of the input speech to digitized speech parameters representative of the input voice signal using Linear Predictive Coding (LPC) techniques. In the exemplary embodiment, the variable rate data source is a variable rate vocoder as described in detail in U.S. Pat. No. 5,414,796 which is assigned to the assignee of the present invention and is incorporated by reference herein. Variable rate data source 12 provides variable rate packets of data at four possible frame rates 9600 bps, 4800 bps, 2400 bps and 1200 bps, referred to herein as full, half, quarter, and eighth rates. Packets encoded at full rate contain 172 information bits, samples encoded at half rate contain 80 information bits, samples encoded at quarter rate contain 40 information bits and samples encoded at eighth rate contain 16 information bits. Packet formats are shown in FIGS. 2A-2D. The packets regardless of size all are one frame length in duration, i.e. 20 ms. Herein, the terms “frame” and “packet” may be used interchangeably.
The packets are encoded and transmitted at different rates to compress the data contained therein based, in part, on the complexity or amount of information represented by the frame. For example, if the input voice signal includes little or no variation, perhaps because the speaker is not speaking, the information bits of the corresponding packet may be compressed and encoded at eighth rate. This compression results in a loss of resolution of the corresponding portion of the voice signal but, given that the corresponding portion of the voice signal contains little or no information, the reduction in signal resolution is not typically noticeable. Alternatively, if the corresponding input voice signal of the packet includes much information, perhaps because the speaker is actively vocalizing, the packet is encoded at full rate and the compression of the input speech is reduced to achieve better voice quality.
This compression and encoding technique is employed to limit, on the average, the amount of signals being transmitted at any one time to thereby allow the overall bandwidth of the transmission system to be utilized more effectively to allow, for example, a greater number of telephone calls to be processed at any one time.
The variable rate packets generated by data source 12 are provided to packetizer 13 which selectively appends cyclic redundancy check (CRC) bits and tail bits. As shown in FIG. 2A, when a frame is encoded by the variable rate data source 12 at full rate, packetizer 13 generates and appends twelve CRC bits and eight tail bits. Similarly, as shown in FIG. 2B, when a frame is encoded by the variable rate data source 12 at half rate, packetizer 13 generates and appends eight CRC bits and eight tail bits. As shown in FIG. 2C, when a frame is encoded by the variable rate data source 12 at quarter rate, packetizer 13 generates and appends eight tail bits. As shown in FIG. 2D, when a frame is encoded by the variable rate data source 12 at eighth rate, packetizer 13 generates and appends eight tail bits.
The variable rate packets from packetizer 13 are then provided to encoder 14 which encodes the bits of the variable rate packets for error detection and correction purposes. In the exemplary embodiment, encoder 14 is a rate 1/3 convolutional encoder. The convolutionally encoded symbols are then provided to a CDMA spreader 16, an implementation of which is described in detail in U.S. Pat. Nos. 5,103,459 and 4,901,307. CDMA spreader 16 maps eight encoded symbols to a 64 bit Walsh symbol and then spreads the Walsh symbols in accordance with a pseudorandom noise (PN) code.
Repetition generator 17 receives the spread packets. For packets of less than full rate, repetition generator 17 generates duplicates of the symbols in the packets to provide packets of a constant data rate. When the variable rate packet is half rate, then repetition generator 17 introduces a factor of two redundancy, i.e. each spread symbol is repeated twice within the output packet. When the variable rate packet is quarter rate, then repetition generator 17 introduces a factor of four redundancy. When the variable rate packet is eighth rate then repetition generator 17 introduces a factor of eight redundancy.
Repetition generator 17 provides the aforementioned redundancy by dividing the spread data packet into smaller sub-packets referred to as “power control groups”. In the exemplary embodiment, each power control group consists of 6 PN spread Walsh Symbols. The constant rate frame is generated by consecutively repeating each power control group the requisite number of times to fill the frame as described above.
The spread packets are then provided to a data burst randomizer 18 which removes the redundancy from the spread packets in accordance with a pseudorandom process as described in copending U.S. patent application Ser. No. 08/291,231, filed Aug. 16, 1994 assigned to the assignee of the present invention. Data burst randomizer 18 selects one of the spread power control groups for transmission in accordance with a pseudorandom selection process and gates the other redundant copies of that power control group.
The packets are provided by data burst randomizer 18 to finite impulse response (FIR) filter 20, an example of which is described in U.S. patent application Ser. No. 08/194,823, and assigned to the assignee of the present invention. The filtered signal is then provided to digital to analog converter 22 and converted to an analog signal. The analog signal is then provided to transmitter 24 which upconverts and amplifies the signal for transmission through antenna 26.
FIG. 3 illustrates pertinent components of a mobile telephone 28 or other mobile station receiving the transmitted signal. The signal is received by antenna 30, downconverted and amplified, if necessary, by receiver 32. The signal is then provided to frame rate detection unit 33 which subdivides the signal into packets and determines the corresponding frame rate for each packet. The frame rate may be determined, depending upon the implementation, by detecting the duration of individual bits of the frame. The packet and a signal identifying the detected frame rate for the packet are then forwarded to CRC unit 34 for performing cyclic redundancy checks or related error detection checks in an attempt to verify that no transmission errors or frame rate detection errors occurred. A frame rate detection error results in the packet being sampled at an incorrect rate resulting in a sequence of bits that are effectively random. A transmission error typically results in only one or two bits being in error. Usually, if a transmission error or frame rate detection error occurs, the CRC unit detects the error. “Bad” frames failing the CRC are erased or otherwise discarded by frame erasure unit 36. “Good” frames which pass the CRC are routed to variable rate decoder 40 for conversion back to digitized voice signals. The digitized voice signals are converted to analog signals by a digital to analog converter (not shown) for ultimate output through a speaker 42 of the mobile telephone.
Depending upon the implementation, no separate frame erasure unit 36 is necessarily required. Rather, CRC unit 34 may be configured merely to not output bad frames to variable rate decoder 40. However, provision of a frame erasure unit facilitates generation of frame erasure signals for forwarding back to the base station to notify the base station of the frame erasure error. The base station may use the framer erasure information to modulate the amount of power employed to transmit signals perhaps as part of a feedback system intended to minimize transmitted power while also minimizing frame errors.
As noted above, by varying the frame rate of packets to thereby compress the information contained therein, the overall bandwidth of the system is utilized more effectively, usually without any noticeable effect on the transmitted signal. However, problems occur occasionally which have a noticeable effect. One such problem occurs if a frame subject to a frame rate detection error or a transmission error nevertheless passes the CRC. In such case, the bad frame is not erased but is processed along with other good frames. The error may or may not be noticeable. For example, if the error is a transmission error wherein only one or two bits of encoded speech are in error, the error may have only an extremely slight and probably unnoticeable effect on the output voice signal. However, if the error is a frame rate detection error, the entire packet will thereby be processed using the incorrect frame rate causing effectively random bits to be input to the decoder likely resulting in a noticeable artifact in the output voice signal. For some systems, it has been found that incorrect frame rate detections occur with a probability of about 0.005% yielding an incorrectly received packet and a corresponding artifact in the output voice signal about every sixteen minutes of conversation time. Although described with respect to a CDMA system using TIA/EIA IS-95-A protocols, similar problems can occur in almost any transmission system employing variable transmission rates and in related systems as well.
It would be desirable to remedy the forgoing problem, and it is to that end that the invention is primarily drawn.